Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Radio detection and ranging (RADAR) systems can be used to actively estimate distances to environmental features by emitting radio signals and detecting returning reflected signals. Distances to radio-reflective features can be determined according to the time delay between transmission and reception. The radar system can emit a signal that varies in frequency over time, such as a signal with a time-varying frequency ramp, and then relate the difference in frequency between the emitted signal and the reflected signal to a range estimate. Some systems may also estimate relative motion of reflective objects based on Doppler frequency shifts in the received reflected signals. Directional antennas can be used for the transmission and/or reception of signals to associate each range estimate with a bearing. More generally, directional antennas can also be used to focus radiated energy on a given field of view of interest. Combining the measured distances and the directional information allows for the surrounding environment features to be identified and/or mapped. The radar sensor can thus be used, for instance, by an autonomous vehicle control system to avoid obstacles indicated by the sensor information.
Some example automotive radar systems may be configured to operate at an electromagnetic wave frequency of 77 Giga-Hertz (GHz), which corresponds to millimeter (mm) electromagnetic wave length (e.g., 3.9 mm for 77 GHz). These radar systems may use antennas that can focus the radiated energy into beams in order to enable the radar system to measure an environment with high accuracy, such as an environment around an autonomous vehicle. Such antennas may be compact (typically with rectangular form factors; e.g., 1.3 inches high by 2.5 inches wide), efficient (i.e., there should be little 77 GHz energy lost to heat in the antenna, or reflected back into the transmitter electronics), and cheap and easy to manufacture.
In some scenarios, efficiency may be difficult to balance with cheap and easy manufacture. Some cheap and easy to manufacture options may involve integrating an antenna into a circuit board (e.g., with a “series-fed patch array”), which is used by many off-the-shelf automotive radars. However, such antennas may lose much of their energy into heating up the substrate of the circuit board. Antennas with the lowest loss may include all-metal designs, but typical all-metal antennas, such as slotted waveguide arrays, can be difficult to manufacture with the small geometries compatible with 77 GHz operation.